Although most major food allergies begin in childhood, one allergy in particular stands apart—shellfish. An allergy to shellfish such as shrimp, lobster, oysters, crab, and scallops may develop any time during a person's life, but tends to present in adulthood. Along with fish, shellfish allergies are the most common adult-onset food allergies. It’s estimated that between 2 and 3 percent of American adults suffer from allergies to one or both.
When compared to other allergies, a shellfish allergy is different in other ways as well. For example, allergic reactions to shellfish are unpredictable, sometimes occurring long after a person has consumed the allergen and has shown no other symptoms. In addition, allergic reactions to shellfish often become more severe with each exposure.
Shellfish allergies are most often a response of the body's immune system to tropomyosin, a protein found in shellfish muscles. Antibodies trigger the release of chemicals such as histamines to attack the tropomyosin, leading to a number of symptoms which can range from mild to life-threatening and tend to lean toward the severe. Although it may take some time for symptoms to present after eating shellfish, most develop within minutes. Symptoms of a shellfish allergy may include:
- tingling in the mouth
- abdominal pain, nausea, diarrhea, or vomiting
- congestion, trouble breathing, or wheezing
- skin reactions including itching, hives, or eczema
- swelling of the face, lips, tongue, throat, ears, fingers, or hands
- lightheadedness, dizziness, or fainting
In the most serious cases, a severe, life-threatening allergic reaction known as anaphylaxis may occur. An anaphylactic reaction requires immediate medical attention. Symptoms of anaphylaxis include:
- a swollen throat (or lump in the throat) that makes breathing difficult
- rapid pulse
- extreme dizziness or loss of consciousness
- a severe drop in blood pressure (shock)
There is currently no cure for a shellfish allergy. The best treatment is to avoid foods such as shrimp, lobster, crab, and other crustaceans. When eating in a restaurant, ask the staff how food is prepared, as a shellfish-based broth or sauce may trigger an allergic reaction.
Many healthcare providers also recommend that people with shellfish allergies carry epinephrine (EpiPen) for self-administration in case of accidental ingestion. Epinephrine (adrenalin) is the first-line treatment for anaphylaxis.
Although deaths from an anaphylactic reaction from eating shellfish are rare, they’re more common than in other food allergies. Most doctors agree that a person who has both a shellfish allergy and asthma should have an EpiPen on hand in case of an emergency.
Shellfish Allergy and Iodine
There has been some confusion in recent years regarding the relationship between shellfish allergy and iodine. Specifically, many people falsely believe that iodine (which is often used in medications and contrast media) may trigger an allergic reaction in people with a shellfish allergy. The misconception is largely related to a Florida case in which the family of a man who died from a severe allergic reaction was awarded a $4.7 million settlement after successfully arguing in court that the contrast iodine used in his treatment for acute coronary syndrome had caused the man's death.
Iodine is an element found throughout the body and is essential to the production of thyroid hormones as well as various amino acids. In short, human beings cannot survive without it. A study published in 2010 in the Journal of Emergency Medicine concluded that iodine is not an allergen. According to the researchers, "allergies to shellfish, in particular, do not increase the risk of reaction to intravenous contrast any more that of other allergies."
Shellfish Allergy Test
A simple skin prick test can identify a shellfish allergy. The test involves puncturing the skin of the forearm and introducing a small amount of the allergen into it. If a person is allergic, a small itchy red spot will appear within a few minutes as the mast cells release histamine.